Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method or an electrostatic recording method.
Description of the Related Art
Conventionally, image forming apparatuses such as copiers, printers and facsimile apparatuses using an electrophotographic method or an electrostatic recording method have been known. In such image forming apparatuses, a toner image formed on a photosensitive drum is transferred by a transfer roller onto a recording material (e.g., paper) that is being conveyed, whereby an image is formed. Here, on a surface of the photosensitive drum, some toner remains without being transferred. Thus, a technique including a cleaning blade made of an elastic member, the cleaning blade scraping off toner remaining on a photosensitive drum (remaining toner) is known.
The cleaning blade includes a sharp edge at a tip thereof provided in pressure contact with the surface of the photosensitive drum on the downstream side relative to a transfer site in a direction of rotation of the photosensitive drum to scrape remaining toner off. Thus, a large friction force may be generated between the edge at the tip of the cleaning blade and the photosensitive drum. In other words, if there is a certain amount of toner in a contact portion between the cleaning blade and the photosensitive drum, the toner or an external additive in the toner functions as a lubricant, and thus, only a small friction force is generated. However, if there is no or only a small amount of toner in the contact portion, a large friction force is generated between the edge at the tip of the cleaning blade and the photosensitive drum. Consequently, problems such as bur formation and stick-slip in the cleaning blade are likely to occur. In particular, if the photosensitive drum is a new one, there is no toner in the contact portion between the cleaning blade and the photosensitive drum and thus such problems are more likely to occur.
In order to solve such problems, Japanese Patent Application Laid-Open No. 2004-191737 discloses a technique in which toner is supplied to a contact portion between a cleaning blade and a photosensitive drum when no image formation is performed.
In general, a transfer roller is often configured so as to be rotated to follow a photosensitive drum. In this case, if a friction force between the transfer roller and the photosensitive drum is excessively low, the transfer roller slips. When processing for supplying toner to a contact portion between a cleaning blade and the photosensitive drum is performed during no image formation being performed as mentioned above, the toner passes through between the photosensitive drum and the transfer roller. Thus, the amount of toner between the photosensitive drum and the transfer roller becomes large, and the friction force between the photosensitive drum and the transfer roller thereby becomes excessively low, which may result in slippage of the transfer roller. If the transfer roller slips, a part of the transfer roller takes a scraping of the toner and the transfer roller remains in a state in which toner adheres thereto. In this case, in subsequent image formation operation, the problem of toner adhering to a back surface of a recording material such as paper occurs.
As a countermeasure for this problem, it is possible to, when processing for supplying toner to a contact portion between a cleaning blade and a photosensitive drum, reduce a thickness of toner formed on the photosensitive drum to reduce the toner amount. However, because of variations in, e.g., output of high-voltage power supply such as a developing bias and sensitivity of the photosensitive drum, it is very difficult to control the toner amount so as to prevent both transfer roller slippage and cleaning blade bur formation. It is also possible to prevent slippage of the transfer roller by driving the transfer roller itself to rotate. However, e.g., a driving gear is required to drive the transfer roller, resulting in increase in cost of the apparatus.